Monitoring Natural Low-Frequency/Medium-Frequency/High- Frequency Radio Emissions from Remote Sites A.T

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Monitoring natural low-frequency/medium-frequency/high- frequency radio emissions from remote sites A.T. WEATHERWAX, I. LABELLE, M.L. TRIMPI, and R. BRITTAIN, Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire 03755

ince the late 1940s, ground-based observations of natural cycle. To gain a better understanding of auroral LF/MF/HF S radio emissions have been reported in the frequency range emissions, a ground-based effort has been started in Antarcti- 0.10-10.0 megahertz (MHz) [hereafter referred to as the low- ca to study this much neglected part of the spectrum. Cur- frequency/medium-frequency/high-frequency (LF/MF/HF) rently, radio receivers are being operated at South Pole Sta- band]. Reported phenomena do not fit the known features of tion (740 magnetic) and at the first U.S. automatic geophysical very-low-frequency (VLF) hiss or auroral kilometric radiation observatory (AGO) site called P2 (approximately 70 0 magnet- (AKR). At tens of megahertz, the phenomena observed on the ic). The South Pole receiver has been operating since the 1992 ground include anomalous spikes in riometer measurements season, whereas the AGO receiver was deployed just last year; up to several decibels above the background, reportedly asso- AGO data should be available for analysis in winter 1993. ciated with longer duration absorption events (Harang 1969; Operation of the receivers in Antarctica not only will comple- Nishimuta, Ose, and Sinno et al. 1969). At several megahertz, ment our work currently underway in the Northern Hemi- Kellogg and Monson (1979, 1984) reported banded emissions sphere (Alaska) but also should provide a lower noise envi- (which they call auroral roar) near twice the ionospheric elec- ronment for observations since manmade interference tron cyclotron frequency (fce) - should be minimal. More recently, we have confirmed the observations of Figure 1 shows a block diagram of the Dartmouth Kellogg and Monson (1979, 1984) and have observed emis- LF/MF/HF receiver. The antenna used at both locations is a sions near 3ce (Weatherwax et al. 1993). Observers have also nonresonant vertical dipole [approximately 3.5 meters (m)]. reported broadband emissions at a few megahertz which This antenna is easy to transport and deploy, requires little might be associated with auroral synchrotron/ cyclotron radi- area at a site, and requires no maintenance or tuning after it ation (for example, Berkey and Parthasarathy 1964; has been erected. The receiver can be controlled by either a Parthasarathy and Berkey 1964; Weatherwax et al. in press). At personal computer (PC) or an electrically erasable program- lower frequencies, wideband noise associated with aurora has mable read-only memory chip (EEPROM). A PC controls the been reported from Alaska (Benson et al. 1988) and Norway receiver at the South Pole, but an EEPROM controls the AGO (Benson and Desch 1991), based on measurements at four receiver because power constraints at the AGO site make the fixed frequencies between 150-1,500 kilohertz (kHz). Various use of a PC impossible. The local oscillator signal is used to observations relevant to a ground-based study of auroral tune the receivers. A board located in the PC at the South Pole radio noise are reviewed by Ellyett (1969), LaBelle (1989), and LaBelle and Weatherwax (in press). In addition to the observational evidence mentioned above, theoretical work suggests the possible existence of Data Acquisition Unit A/D Converter jpersonai Computer auroral-related LF/MF/HF radio emissions. Most recently, (AGO Only) (South Pole Only) (South Pole Only) Wu, Yoon, and Freund (1989) proposed that electrons trapped below a parallel potential drop in the auroral region could give rise to downward-propagating electron cyclotron waves Local Receiver Board by a resonant mechanism similar to the cyclotron maser l.4_^ Oscillator 1.4 ...... mechanism proposed for AKR (Wu and Lee 1979). Ziebell, Wu, and Yoon (1991) have applied this idea to a realistic model of the auroral zone and find significant amplification Dipole Antenna of downward-propagating waves peaking at frequencies of a few hundred kilohertz, consistent with ground-based obser- Preamplifier vations by Benson et al. (1988). Synchrotron/ cyclotron radiation, transitional radiation, Cherenkov radiation, plasma oscillations, and the generation of whistler-mode noise by Calibration Signal trapped electrons could also be possible sources of Signal from Antenna LF/MF/HF emissions. The published observations of auroral LF/MF/HF emissions have yet to determine fully the frequency-time charac- Figure 1. Block diagram of the LF/MF/HF receiving system devel- oped at Dartmouth College and deployed in the first U.S. automatic teristics, generation mechanisms, or the occurrence statistics geophysical observatory and at South Pole Station. (ND denotes of these events as a function of latitude, longitude, and solar analog/digital.)

ANTARCTIC JOURNAL - REVIEW 1993 303 South Pole Station 5—July-92

4500.0 -

4000.0 -

• 35000 N - - - ...... -...... 30000 ..-.

2500.0 -

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1500.0 - Figure 2. Gray scale display of 30 1000.0 - minutes of LF/MF/HF data (0.3-4.8 MHz) recorded 5 July 1992, at South Pole Station. White 500.0 - pixels correspond to 7 nV/mVHz; black pixels to 70 nV/mvHz. Ris- ing tones at 0430-0432 UT and 0452-0500 UT are certainly of CC/,C CC4 a. manmade origin, though their .J . .4 ...... 0 .00 exact source is unknown. The ver- 0 tical bands (for example, at 0434 Time (UT) UT and 0454 UT) are also artificial. digitally synthesizes this signal, whereas a phase-locked loop is used in the AGO receiver. Programmability at both sites allows for a large number of operational measurement modes: different sample rates at different times of day, changes in which frequencies are sam- B und Noise Comparison 10-_ pled, calibration pulses at different intervals, and so forth. Two Rivers, Alaska (thick line) Among other advantages, this flexibility means that the fre- (21 -MAR-92 @ 1400 UT) quencies recorded can be customized for a specific site, for example, by stepping around known sources of interference. In the case of the AGO receivers, the timing as well as data sampling and archiving are handled by the data-acquisition 10 unit. Data are stored on optical disks and distributed to exper- imenters once each year. At South Pole Station, data archiving and timing are handled by the PC, needing only minor assis- tance by an operator. E Figure 2 shows 30 minutes of data recorded on 5 July 1992 at South Pole Station. Weak rising tones at 2.6-2.7 MHz near 0430-0432 universal time (UT) and 0452-0500 UT and continuous dark horizontal lines (for example, approximately 2 MHz) are believed to be due to local manmade interference, South Pole (thin line) though the source is unknown. The vertical stripes near 0434 (05-JUL-1992 @ 0200 UT) and 0454 UT result from a periodic artificial disturbance. 10 Besides these features, the spectrum is remarkably uniform 1000 2000 3000 4000 480 for the half hour displayed. In fact, at the South Pole, no nat- Frequency (kHz) ural signals have been definitely identified, even though iden- tical receiving systems in Alaska have revealed emissions. Not Figure 3. Line spectra from quiet times recorded at South Pole Sta- all of the apparently artificial signals seen in the data have tion (thin line) and Two Rivers, Alaska (thick line).

ANTARCTIC JOURNAL - REVIEW 1993 304 been associated with known local (station) or distant sources, based instruments such as magnetometers, VLF receivers, however. riometers, and all sky cameras. Figure 3 shows individual spectra recorded during quiet This research was funded by National Science Founda- periods in Alaska (thick line) and the South Pole (thin line). tion grant OPP 89-15635 to Dartmouth College. Over most of the LF/MF band (f<1 MHz) and at many frequencies above 1 MHz, the noise level at South Pole lies References below that at Alaska. Hence, the receiver is sufficiently sensi- tive to record the phenomena observed in Alaska (Weather- Benson, R.F., and M.D. Desch. 1991. Wideband noise observed at wax et al. 1993, in press). Over the entire spectrum measured ground level in the aurora! region, Radio Science, 23(4), 943. in Alaska and over most of the spectrum measured at the Benson, R.F., M.D. Desch, R.D. Hunsucker, and G.J. Romick. 1988. Ground level detection of !ow and medium frequency auroral South Pole, the sensitivity of the receiver is determined by radio emissions. Journal of Geophysical Research, 93(A1), 277. manmade noise in the environment rather than the instru- Berkey, F.T., and R. Parthasarathy. 1964. The rare instances of period- ments noise level. Both the South Pole and Alaska receivers ic emission of synchrotron radiation from the auroral electrons. were calibrated in New Hampshire using a parallel-plate Journal ofAtmospheric and Terrestrial Physics, 26(9), 936. antenna of known characteristics to detect the continuous E1!yett, C.D. 1969. Radio noise of auroral origin. Journal ofAtmospher- ic and Terrestrial Physics, 31(5), 671-682. 100-kHz loran navigational beacon transmitted from Maine. in Harang, L. 1969. Radio noise from the aurora. Planetary and Space An effective antenna height of 2.4 used in calculating Science, 17(5), 869-877. field strength. Kellogg, P.J., and S.J. Monson. 1979. Radio emissions from the aurora. Kellogg and Monson (1984) reported 2ce emissions on Geophysical Research Letters, 6(4), 297. about half of 40 nights of observations at Churchill, Manitoba Kellogg, P.J., and S.J. Monson. 1984. Further studies of auroral roar. Radio Science, 19(2), 551. (66.3° magnetic). By contrast, Weatherwax et al. (1993) LaBelle, J. 1989. Radio noise of auroral origin: 1968-1988. Journal of observed similar emissions less than 10 percent of the time Atmospheric and Terrestrial Physics, 51(3), 197. from Two Rivers, Alaska, and no events were observed from LaBelle, J., and A.T. Weatherwax. In press. Ground based observation the South Pole. We suspect that the absence of events at the of LF/MF/HF radio waves of auroral origin. In T. Chang (Ed.), South Pole is a consequence of its higher latitude. The Alaska Physics of space plasmas. Nishimuta, I., M. Ose, and K. Sinno. 1969. Abnormal enhancements of and Churchill sites lie under the auroral oval near local mid- HF noise intensity at Syowa Station, Antarctica. Journal of Geo- night, whereas the South Pole statistically lies under the oval magnetics and Geoelectricity, 21(3), 697. near local noon, a less active time. The differences in event Parthasarathy, R., and F.T. Berkey. 1964. Radio noise from the auroral rate at the different sites, however, could also be a function of electrons. Journal of Atmospheric and Terrestrial Physics, 26(2), magnetic activity; existing data sets are not yet sufficient to 199-203. Weatherwax, A.T., LaBelle, M. Trimpi, and R. Brittain. 1993. resolve this issue. Future sites at different latitudes in Antarc- J. Ground-based observations of radio emissions near 21ce and 3fce tica should help resolve some of the outstanding problems in the auroral zone. Geophysical Research Letters, 20(14), concerning these emissions. 1447-1450. Operating a receiver in Antarctica complements our work Weatherwax, A.T., I. LaBelle, M. Trimpi, R. Brittain, and R.A. in the Northern Hemisphere by providing increased sensitivi- Treumann. In press. Ground based observations of MF/HF radio noise in the auroral zone. Journal of Geophysical Research. ty as well as indicating the global nature of LF/MF/HF emis- Wu, C.S., and L.C. Lee. 1979. A theory of the terrestrial kilometric radi- sions. In the long run, we hope to assess whether it will be ation. Astrophysical Journal, 230(2), 621-626. possible to use these signals to remotely sense features of the Wu, C.S., P.H. Yoon, and H.P. Freund. 1989. A theory of electron auroral electrons. Such ground-based measurements would cyclotron waves generated along auroral field lines observed by complement rocket and satellite techniques in the following ground facilities. Geophysical Research Letters, 16(12), 1461. Ziebell, L.F., C.S. Wu, and P.H. Yoon. 1991. Kilometric radio waves ways: low cost, ability to image a large area, and ability to col- generated along auroral field lines observed by ground facilities: lect continuous data rather than occasional snapshots. Such A theoretical model. Journal of Geophysical Research, 96(A2), measurements would also complement existing ground- 1495.

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